This invention relates in general to semiconductor chip bonding techniques and apparatuses and in particular, to a system for securing and electrically connecting a semiconductor chip to a substrate. In conventional packaging technology, a semiconductor chip is usually first enclosed within a semiconductor package, and the package in turn connected to a large substrate such as a printed circuit board. To provide electrical connections between the semiconductor chip and the printed circuit board, the chip is first electrically connected to a carrier, substrate or a package and the carrier, substrate or the package then in turn connected electrically to conductive traces on the board. For example, in many conventional chip packages, bonding wires have been used to connect selected points on a semiconductor chip to bonding fingers on the surface of a substrate used to support the chip.
The above-described system for mounting a semiconductor chip on a printed circuit board is disadvantageous for a number of reasons. In many applications where space on the printed circuit board is at a premium, such as in notebook or laptop computers or those used in many military and space applications, the above-described system is wasteful of space on the printed circuit board. Furthermore, by first enclosing the semiconductor chip within an intermediate substrate to form a semiconductor package, the package designer would have to find a way to dissipate the heat generated by the chip enclosed by the substrate. Where the substrate is made of a poor heat conductor such as plastic, the heat generated can be a significant problem.
Semiconductor chip packages typically are provided with pins at the outside surfaces of the package where the pins are electrically connected through vias or fingers and bonding wires to the semiconductor chip inside the package. The pins are then connected to conductive traces on the printed circuit board by a conventional method such as soldering. The pitch between adjacent pins or leads of a semiconductor package is typically in the range of 25-100 mils where the pins or leads themselves typically have diameters or lead widths in the range of 6-20 mils.
With the advent of miniaturized devices such as laptop or notebook computers or in military and space applications, it is desirable to provide denser electrical connections between the chip and the printed circuit board. The above-described conventional package system limits how dense these connections can be. It is therefore desirable to provide chip bonding techniques that permit denser spacing of electrical connections then permitted by conventional techniques.
One way to provide denser electrical connections between the chip and the printed circuit board is to directly bond the semiconductor chip to the board without using an intermediate substrate such as a semiconductor package. One such conventional packaging system that has been used is known as "flip-chips," where solder bumps have been used to bond selected points on the semiconductor chip directly to bonding pads or conductive traces on the printed circuit board or to a lead frame. In order to connect the chip to the printed circuit board, the solder bumps are heated until they reflow so as to electrically connect and physically attach the semiconductor chips to the board. When the solder bumps reflow, they are in a semifluid state. To avoid any bridging which causes shorts, the solder bumps cannot be placed too close together so that a minimum spacing between bond pads of 5-8 mils must be maintained between adjacent electrical connections in the "flip-chip" system. Furthermore, the material that the printed circuit board is composed of may have a different thermal expansion coefficient compared to the material of the semiconductor chip. Typically, the substrate material in the board would expand more than silicon. In the "flip-chip" system, the silicon material in the chip is spaced from the substrate by a distance of the order of 1 or 2 mils. Therefore, when a chip and a substrate bonded through the "flip-chip" system experience temperature variations, the solder joint between the chip and the substrate will be subjected to stresses and may break. Even if the solder joint between the chip and the substrate does not break right away when subjected to temperature variations, the cumulative effect of the variations and the resulting stresses over an extended period of time may cause such solder joint to break, thereby causing future reliability problems. This is true particularly where the solder joint is cooled from room temperature to below freezing temperatures.
None of the above-described packaging systems is entirely satisfactory. It is therefore desirable to provide an improved system for securing and electrically connecting a semiconductor chip to a substrate.